1. Field of the Invention
The present invention relates to light sources. More specifically, this invention pertains to a highly stable broadband light source based on a semiconductor light source and an optical filter and a method for stabilization of such a broadband light source.
2. Description of the Prior Art
Fiber-optic gyroscopes have been used for many years in medium-accuracy navigation systems. Their use in inertial navigation systems (INS), however, has been limited by the low scale factor accuracy (>100 units per million (ppm)) that results primarily from the wavelength stability of the light source.
Highly stable superfluorescence light sources having residual wavelength inaccuracies of 0.05 units per million (ppm) per degree Celsius (0.05 ppm/° C.) are known. However, they are very expensive and employed in only a small number of high-accuracy INS systems.
Superluminescence diodes (SLDs) offer an inexpensive alternative semiconductor light source. They, however, have two significant disadvantages. First, wavelength temperature dependence is approximately |dλ/dT|=400 ppm/° C. Secondly, they age, producing undesirable wavelength drifts and, thus, poor accuracy and overall stability.
The use of Bragg gratings to improve the wavelength stability of erbium-doped superluminescence fiber sources is described, for example, in N. Cerre et al., “Fiber Bragg Grating For Use Within High-Accuracy Fiber Optic Gyroscope”, Proc. OFS (1997). In this context, while the stability and low temperature dependence of the grating are utilized inherently, the possible results are not by themselves sufficient for use in conjunction with a semiconductor light source (e.g. a SLD).
Moreover, printed document DE 198 02 095 C1 discloses a method and device for the stabilization of the scale factor of an optical gyroscope. In the disclosed method, the emission bandwidth of a light source irradiating light into the end of a fiber is limited in the wavelength region of the intensity characteristic of the light to a much smaller transmission bandwidth by a narrowband optical transmission filter. The transmission bandwidth of the optical filter is thus actively changed in dependence of a temperature at the filter by a precalibrated look-up table. See, in addition, DE-A-31 48 925.